Don't forget to clear your lines when you are done with them.
Don't just keep drawing the same line over and over either. Because
it will slow down the player eventually.

The trick is that you need to pick which movieclip you want
to draw in and then carefully choose your x and y. You also have to
keep the depths of the movieclips in mind so that your line is in
the right stacking order and isn't on top of things you don't want
it on top of.